As I don't want to ruin the experience of those reading or planning to read this (wonderful) series, the question will be in spoiler mode and title is quite vague on purpose. Hover your mouse to see the question.
Do not do it if you are reading or plan to read the books!
So here we go:
When Roland enters the last room of the Dark Tower, he starts again, almost from scratch (The wheel of Ka turns...) but with the slight difference of now having Eld's Horn with him.
I found several references to when Cuthbert let the Horn and when Roland wakes up after dreaming about this (mainly in Wolves of the Calla).
Roland will let it lie in the dust. In his grief and bloodlust he will forget all about Eld's Horn.
Roland awoke from another vile dream of Jericho Hill in the hour before dawn. The horn. Something about Arthur Eld's horn.
... Roland agreed, thinking of Jericho Hill. Thinking of the fallen horn.
It's quite clear the horn is important and we understand why at the end ans the seventh book but I don't get how it will make things different as Roland will start again his journey, as I don't remember of any situation where Roland misses the Horn and where things would have gone differently with it. There should be one but I don't find it